1. Field of the Invention
The present invention relates to a novel preparation process of isoimide comprising reacting in the presence of a haloiminium salt and basic substance.
2. Related Art of the Invention
Phenylmaleisoimide, N,N-phenylenebisisomaleimide, phenylisophthalimide and other isoimide monomers are useful compounds which are known as a raw material monomers and intermediate of agricultural chemicals and medicines. Preparation process of these isoimide has been conventionally known. For example, U.S. Pat. No. 2,998,429 has disclosed a process for ring-closing maleamic acid and converting to a corresponding isoimide isomer by using triethylamine in the presence of dichloroacetyl chloride. U.S. Pat. No. 3,035,065 has described a process for converting to a corresponding isoimide isomer in the presence of dicyclohexylcarbodiimide. However, these processes have a problem of high cost.
Further, as an improvement of these processes, Japanese Laid-Open Patent SHO 52-97959 has disclosed a preparation process of corresponding isoimide by reacting amic acid with ketene in the presence of acetic anhydride. However, at least an equimolar amount of expensive ketene for amic acid is required in order to form a proper amount of isoimide. Ketene is a very labile compound and thus the above process using ketene is also unfavorable as a preparation process in industry.
It has also been known as a preparation process of isoimide to react amic acid or polyamic acid with acetic anhydride, ethyl chloroformate and triethylamine, trifluoroacetic anhydride and other dehydrating agents. However, any of these processes involve many problems, for example, the reaction has slow velocity and low selectivity, excessive facilities are required for separation and recovery of byproducts, and it is difficult to repeatedly use the recovered substances.
On the other hand, polyisoimide which is known as a polyimide precursor can isomerize with ease to polyimide by heat treatment without evolution of water or other low molecular weight compounds, and is thus a material of being focused attention as a polyimide precursor in place of polyamic acid.
Japanese Laid-Open Patent HEI 4-214728 has disclosed a preparation process of polyisoimide which uses a nontoxic dihydroquinoline derivative as a dehydrating agent and does not require separation of solid byproducts. The process is specifically characterized in the nontoxic dehydrating agent. However, the process also forms four species of byproduct and a quinoline byproduct requires a great labor in order to regenerate the dihydroquinoline derivative. These problems have inhibited reduction in the production cost.